1. Field of the Invention
This invention relates to an improvement in add-drop multiplexes, known also as add-drop multiplexer, also known in Europe as drop-and-insert multiplexes. Such multiplexes are a particular form of combined multiplexer/demultiplexer for digital signals, optimised for the low cost insertion or extraction of a proportion of the "through" traffic capacity of the multiplex.
2. Description of the Related Art
A digital multiplex combines multiple digital signals streams into a composite stream of digits, each of the digits in the original streams conventionally being allocated to a unique timeslot in the frame structure of the composite stream. Following recognition of the inserted frame start signal at the demultiplexer, the demultiplexer separates the multiple signals so as to restore them to their original form.
In a terminal digital multiplex, all of the capacity of the composite stream is accessible in the form of multiple individual streams; by contrast, in an add-drop multiplex this is commonly not the case, and there are other differences which are outlined below.
It is gradually becoming a common feature of add-drop multiplexes that they incorporate a matrix switch. This switch has the ability to control the routing of traffic in small increments, each of which is a small fraction of the total traffic passing through the multiplex.
Where this fraction is particularly small, the switch has a correspondingly high complexity, because of the large number of such increments to be manipulated, and this invention relates to a proposed simplification of the switch.